One example of a "via-holed" semiconductor device and method of making it is shown in FIGS. 1(a) through 1(d).
A plurality of semiconductor elements having source electrodes 2, drain electrodes 3 and gate electrodes 4, respectively, are formed on a front surface of a semiconductor substrate 1. Then, the semiconductor substrate 1 is thinned to a desired thickness by etching or grinding (or any other suitable technique) the substrate 1 from the opposing rear surface. Next, as shown in FIG. 1(a), a first resist coating 21 is used as a mask to etch a large recess 22 in a rear surface region corresponding to the semiconductor elements, particularly the source electrodes 2 in the illustrated example, of the respective semiconductor elements. After the formation of the recess 22, the first resist coating 21 is removed. Then, as shown in FIG. 1(b) and 1(c), a second resist coating 23 is formed, which coating 23 acts as a mask for etching the substrate 1 in the bottom of the recess 22 to form a through-hole 24 extending to each of the source electrodes 2. After that, the second resist coating 23 is removed. Finally, as shown in FIG. 1(d), the rear surface of the substrate 1, the inner surface of the recess 22 and the inner surface of each of the through-holes 24 are coated with a metal layer 25 so that the respective source electrodes 2 and the rear surface of the semiconductor substrate 1 are electrically interconnected.
Another example of conventional "via-holed" semiconductor devices is disclosed in Japanese Published Patent application 63-155673 published on June 28, 1988. The semiconductor device of this publication has a structure as shown in FIG. 2, which is fabricated by thinning a substrate 1 to a thickness on the order of 30,.mu.m, forming individual stepped through-holes 30 one for each source electrode 2, and thereafter coating the inner surfaces of the through-holes 30 and the rear surface of the substrate 1 with a metal layer 31.
According to the conventional method of making a via-holed semiconductor device shown in FIG. 1, because the large recess 22 must be first formed by the first etching process before etching the through-holes 24, a large step D is formed between the rear surface of the substrate 1 and the bottom surface of the recess 22. That bottom substrate is coated with the second resist coating 23 acting as a mask for forming the through-holes 24. Because of this step D, it is difficult to form the second coating 23 with a uniform thickness. This step, furthermore, prevents accurate exposure of the second resist coating 23 to a pattern of radiation for patterning the coating 23.
As for the semiconductor device shown in FIG. 2, because the substrate 1 must be thinned to a small thickness on the order of 30 .mu.m, there is a decrease in strength of the substrate and, when the device is a monolithic microwave integrated circuit device, an increase in transmission loss between microstrips. Further, since in the conventional semiconductor device of FIG. 2 the through-holes 30 are individually formed for each source electrode 2, the thermal resistance of the device is undesirably large.
Therefore, the object of the present invention is to eliminate the above-stated problems.